Direct In Situ Measurement of Quantum Efficiencies of Charge Separation and Proton Reduction at TiO2-Protected GaP Photocathodes

Photoelectrochemical solar fuel generation at the semiconductor/liquid interface consists of multiple elementary steps, including charge separation, recombination, and catalytic reactions. While the overall incident light-to-current conversion efficiency (IPCE) can be readily measured, identifying the microscopic efficiency loss processes remains difficult. Here, we report simultaneous in situ transient photocurrent and transient reflectance spectroscopy (TRS) measurements of titanium dioxide-protected gallium phosphide photocathodes for water reduction in photoelectrochemical cells. Transient reflectance spectroscopy enables the direct probe of the separated charge carriers responsible for water reduction to follow their kinetics. Comparison with transient photocurrent measurement allows the direct probe of the initial charge separation quantum efficiency (ϕCS) and provides support for a transient photocurrent model that divides IPCE into the product of quantum efficiencies of light absorption (ϕabs), charge separation (ϕCS), and photoreduction (ϕred), i.e., IPCE = ϕabsϕCSϕred. Our study shows that there are two general key loss pathways: recombination within the bulk GaP that reduces ϕCS and interfacial recombination at the junction that decreases ϕred. Although both loss pathways can be reduced at a more negative applied bias, for GaP/TiO2, the initial charge separation loss is the key efficiency limiting factor. Our combined transient reflectance and photocurrent study provides a time-resolved view of microscopic steps involved in the overall light-to-current conversion process and provides detailed insights into the main loss pathways of the photoelectrochemical system.


S1 Materials Preparation
The Zn-doped 100-oriented GaP single crystals have a dopant concentration of ~6.4×10 16 cm -3 (determined by impedance spectroscopy, see below and Fig S1 for detail).
The electrode was prepared according to a previous procedure. [1][2] In brief, the TiO 2 layer was deposited by atomic layer deposition (ALD) at 250 C with TiCl 4 as titanium source and water vapor as oxygen source. The ALD TiO 2 was verified to be of anatase phase with Raman spectroscopy in previous study. 3 A Ga-In alloy was painted to the back of the GaP for ohmic contact which is later connected to the copper conductive tape and sealed with epoxy to form an electrode for study.
The ALD TiO 2 protected GaP demonstrated excellent stability under CO 2 reduction conditions (>8h measured) while enhancing onset potential and photocurrent. 1-2 A previous photoelectrochemical study of ALD TiO 2 protected p-GaP photocathodes reported that the light-induced hydrogen evolution reaction (HER) performance depends sensitively on the thickness of the TiO 2 layer. [1][2] The onset potential shifts to less negative values, and the photocurrent at the same potential increases with the ALD thickness, reaching the best performance at ~10 nm; further increase of the ALD thickness decreased the performance.
To understand the enhancement mechanism produced by the ALD TiO 2 layer, we have chosen GaP electrodes with 5 nm TiO 2 layer for the simultaneous in-situ transient reflection spectroscopy and photoelectrochemical measurements described in the main text. Two batches of GaP/TiO 2 electrodes were used in this study and they produce similar IPCE performance under CW illumination.
Electron Microscopy: Cross-sectional high-resolution transmission electron microscope (HRTEM) images of the GaP/TiO 2 interface are shown in Figure S1. Here, the TiO 2 film was deposited by atomic layer deposition (ALD). A thick layer of Pt was deposited on the GaP/TiO 2 in order to provide structural stability during the TEM cross-section sample preparation, however, this Pt layer was not used in the photoelectrochemical measurements.
These figures show a TEM cross-sectional image of GaP with a 5 nm thickness (nominal thickness) layer of amorphous TiO 2 . This corresponds to 100 cycles in the ALD process.
From TEM images, we find that the actual thickness is 4.95 nm, in good agreement with the nominal thickness value. While the TiO 2 appears to be predominantly amorphous, some crystal lattice features can be seen near the gallium phosphide interface.  Figure S1. Cross-sectional transmission electron microscopy images of the GaP/TiO 2 interface. Here, the sample is capped with a thick layer of platinum, not used in the photoelectrochemical measurements.
In Figure S2, energy-dispersive X-ray spectroscopy (EDS) was used to spatially the spatial maps corresponding to gallium and phosphorus, respectively. These spatial maps confirm the compositional labeling and interfacial boundaries drawn in Figure S1. Figure S2. Energy-dispersive X-ray spectroscopy (EDS) images of the GaP/TiO 2 interface collected in the scanning transmission electron microscopy (STEM) mode. Here, the sample is capped with a thick layer of platinum, not used in the photoelectrochemical measurements.

S2 Photoelectrochemical Setup
All electrochemical experiments were carried out by a CHI660E workstation (CH Instrument) in a three-electrode setup. A Pt wire acts as a counter electrode and Ag/AgCl  The dopant concentration is determined by Mott-Schottky equation: Here, C is the capacitance, q is the elementary charge,  is the permittivity of GaP, V is applied potential, V fb is the flat band potential, k B is Boltzmann constant and N A is the dopant concentration. The capacitance of the GaP/TiO 2 electrode is measured as a function of applied potential by impedance spectroscopy ( Figure S3). By fitting the capacitancebias data in Figure S3 to Equation S1, the dopant concentration can be determined to be 6.4*10 16 cm -3 with 0.16 cm 2 electrode surface area (4 mm by 4 mm). From the intercept of the fit to constant capacitance as shown in Figure S3, the flat band potential is calculated to be 1.00 V (vs Ag/AgCl) according to: Using the values of dopant density and the flat band potential determined above, the depletion width in GaP as a function of applied potential can be calculated: The electrical field could be described by: The electrical field at the interface is calculated by:  [5][6] When the applied bias approaches the conduction band edge, the electrical field strength will reach its maximum (209 kV/cm) at -1.1 V vs Ag/AgCl. Figure S4a shows the IPCE as a function of applied bias at several illumination power densities, and Figure S4b

S4.1 GaP/TiO 2 reflection loss in PEC cell.
The reflection loss of the GaP/TiO 2 electrode is defined as the ratio of the reflected beam power and the input beam power. The measurement was done in the PEC cell, where both the air/cell interface and water/electrode interface produce a reflection. The reflection loss, as shown in Figure S5, is 6.6% for air/cell reflection and 18.4% for water/electrode interface. In total, the reflection loss is 25%. Thus, 75% incident photon is absorbed by the GaP/TiO 2 . Figure S5. The reflection loss of the air/cell interface (red circle) and water/electrode interface (green triangle) is measured by the slope of the output power to the input power.

S5 IPCE under femtosecond pulsed illumination.
The typical photocurrent transient under pulse laser illumination are shown in Figure   S6a.  Figure S6b. Because of the short laser pulse (150 fs) and low repetition rate (500 Hz) used for the transient reflectance measurement, the peak power is ~6x10 10 times higher than the average power and both are indicated in Figure S6b. At a given illumination power density, IPCE increases at more negative applied biases; and at a fixed applied bias, the IPCE decreases at higher excitation power densities. The highest IPCE is 4.6% at -1.5V under 400 nm pulsed laser excitation with an average excitation power density of 1 mW/cm 2 , significantly smaller than those measured under CW illumination. Lowering the fs pulsed illumination power can increase the IPCE comparable to that under CW illumination as shown in Figure 2d, but such power is insufficient for the transient reflectance spectroscopic measurement.

S6.1 Penetration depth of GaP (100) single crystal
Ellipsometry data from previous study for bare GaP (100) single crystals are used to estimate the penetration depth of the pump and probe beams. 7 The pump penetration depths is estimated to be 138 nm at 3.1 eV by d=1/=/(4, and the probe detection depth is 8~21 nm for 3.1~1.5 eV by d=1/=/(4n), where n and k are the real and imaginary part of the refractive index.

S6.2 FKO fitting function
According to the Gauss law, the field strength at the GaP/TiO 2 interface is given by, ,  is the total transferred charge across the GaP/TiO 2 interface per unit GaP surface area,  is the relative permittivity of GaP and  0 is vacuum permittivity.
Thus, the FKO-induced R/R signal amplitude is linearly proportional to the concentration of electrons that are transferred into TiO 2 , providing a convenient probe of the interfacial charge separation in the GaP/TiO 2 p-n junction. 8 In the weak field and large broadening energy limit, the FKO spectrum can be simplified to a third derivative form. [9][10][11] This condition applies to the GaP/5-nm TiO 2 system we studied, because the sequential oscillation of FKO signal is heavily damped in the high energy probe region (>2.8 eV) in Figure 3. The third derivative form under a constant DC field is: 9-12 Here, two transitions (j=1,2) corresponding for heavy hole and light hole are necessary to describe the FKO signals.  is the phase difference between the real and imaginary part of the refractive index, E g is the band gap,  is the broadening energy, m is 2.5 for a direct transition critical point in GaP. In Equation S7, where  is the effective mass defined for heavy hole and light hole in GaP: 4 therefore, the light-modulated FKO signal can be described as: when E AC is small, , then the above equation can be ( + ) 2 -2 ≈ 2 approximated to: which can be written as Equation S14 for fitting: A j is an arbitrary coefficient to adjust the amplitude.
Combining Equation S14 and Equation S6 leads to the conclusion that the FKO amplitude is proportional to the amount of charge carriers that are separated across the junction under a given E DC . For a specific sample, such as GaP/5-nm TiO 2 , only E DC and E AC change with applied bias and power and all other fitting parameters are materialspecific constants. The fitting parameters are shown in Table S1. The determination of the absolute value of E AC is not possible in this experiment, because of the unknown proportionality constants (A j ) that relate the FKO signal amplitude with the product of . E DC as a function of applied bias can be calculated using Equation S5. The fit allows us to determine the relative values of E AC . Therefore, we set the E AC value to be -1 at -1.5V and the FKO spectra at all potentials are fit globally with potential dependent E AC and potential independent parameters (A 1 , Eg j, ϕ j ,  j ). The universal fitting parameters (same for different bias) for Figure 3c is listed in Table S1 and the values for E DC and E AC are listed in Table 1.

S6.3 Power and bias dependence of TR signal amplitudes and kinetics
The kinetics shown in Figure 3b and 3d are fitted by a multiexponential model, the R/R kinetics can be described as:  Table S2, this time constant decreases with the more negative biases. The amplitude (a 0~a3 ) in Table S2 takes the unit of R/R(*1000).   shown in Figure S8d. The result suggests that at low fluence, the charge separation across the GaP/TiO 2 p-n junction is driven by the built-in field. As the excitation power increases, the higher carrier density screens the built-in electric field in the depletion region (also known as band flattening) and increases the recombination loss of photogenerated charge carriers, giving rise to the saturation behavior at high fluences. [13][14][15][16] same time constant and with different amplitudes. The fitting parameters are listed in Table   S3 below. The fast component ratio decreases from ~90% to ~50% as the power increases. Table S3. Fitting of FKO growth kinetics in Figure S8b Power a 1 /% a 2 /%  1 /ps  2 /ps  3 /ns